The present invention relates to a method and apparatus for automatically determining a transmission order of packets in a local area network (LAN) which uses the CSMA/CD protocol.
The CSMA/CD protocol is an ISO standard for the lower two physical and data link layers (in broad sense), particularly for the data link layer (in narrow sense), of the OSI reference model of LAN developed by the IEEE. All stations on LAN with the CSMA/CD protocol are equally dealt with so that it is insufficient for coping with the situation when communications are required to have a transmission order (priority order) of stations. Methods of communications with a transmission order of data have been proposed, such as disclosed in Japanese Patent Laid-open Publication Nos. 2-149041 and 3-268534.
With these conventional techniques, a given transmission order is obeyed and no technique regarding a means for controlling a transmission order is disclosed therein.
With the technique disclosed, for example, in Japanese Patent Laid-open Publication No. 2-149041, a privileged station having a priority of a transmission process over other stations determines, only from the priority order of a transmission packet of its own station, whether or not the privilege can be exercised. Therefore, there is a possibility that urgent communications by another station is intercepted. As a result, the real time performance, i.e., the urgent-station-first performance of the whole network system may be lowered.